1. Field of the Invention
The present invention relates to a water electrolysis system, and a method for operating the water electrolysis system.
2. Discussion of the Background
Generally, hydrogen is used as fuel gas for power generation reaction in fuel cells. The hydrogen is produced by, for example, a water electrolysis device. The water electrolysis device decomposes water to generate hydrogen (and oxygen) by using a solid polymer electrolyte membrane (ion exchange membrane). An electrolyte membrane electrode assembly is formed by placing electrode catalyst layers on both sides of the solid polymer electrolyte membrane. A unit cell is formed by placing feeders on both sides of the electrolyte membrane electrode assembly.
A plurality of unit cells are stacked on top of each other to form a cell unit. A voltage is applied between the ends of the cell unit in the stacking direction, and water is supplied to the feeder at the anode side. Accordingly, water is decomposed and hydrogen ions (protons) are generated at the anode side of each electrolyte membrane electrode assembly. The generated hydrogen ions travel through the solid polymer electrolyte membrane to the cathode side, where the hydrogen ions combine with electrons to produce hydrogen. At the anode side, oxygen generated together with the hydrogen is discharged from the cell unit together with excess water.
The above-described water electrolysis device is generally installed in a hydrogen supply system for supplying hydrogen to an on-board hydrogen tank of a fuel cell vehicle. For example, Japanese Unexamined Patent Application Publication No. 2007-48599 discloses a hydrogen supplying method for supplying hydrogen to an on-board hydrogen tank of a fuel cell vehicle in a hydrogen supply system. The hydrogen supply system includes a water electrolysis device that electrolyzes water to generate high-pressure hydrogen and a hydrogen supply tank for supplying the hydrogen received from the water electrolysis device to the fuel cell vehicle.
According to this hydrogen supplying method, a first hydrogen pressure in the hydrogen supply tank and a second hydrogen pressure in the on-board hydrogen tank are detected. When it is determined that the difference between the first and second hydrogen pressures is greater than a specified pressure, the hydrogen in the hydrogen supply tank is supplied to the on-board hydrogen tank owing to the pressure difference.
When it is determined that the difference between the first and second hydrogen pressures is less than the specified pressure, the water electrolysis device is operated so as to generate hydrogen while the hydrogen is supplied to the on-board hydrogen tank by the pressure of the generated hydrogen.
Thus, the control of the water electrolysis device is simple and the hydrogen can be reliably and quickly supplied to the on-board hydrogen tank. Accordingly, the hydrogen can be easily and economically supplied to the fuel cell vehicle, and the overall system can be made small and simple.